Mass spectrometer



Sept 4, 1956 M. c. BURK ETAL MASS SPECTROMETER 4 Sheets-Seet l Filed Jan. l0, 1955 ATTORNEYS Sept. 4, 1956 M C, BURK ETAL MASS SPECTROMETER 4 Sheets-Sheet 2 Filed Jan. lO, 1955 INVENTORS M. C. BURK F. W. KARASEK ATTORNEY."

M. Ac. BURK ET AL MASS SPECTROMETER 4 Sheets-Sheet 3 Sept. 4, 1956 F/G. 4 BY ATTORNEYS Sept. 4, 1956 M. c. BURK x-:TAL 2,761,974

MASS SPECTROMETER Filed Jan. l0, 1955 4 Sheets-Sheet 4 5 IN V EN TORS.

M. C. BURK F. W. KARASEK A TTOR/VEVS United States Patent C l MASS SPECTROMETER Marvin C. Burk, Bartlesville, and Francis W. Karasek,

Dewey, Okla., assignors to Phillips Petroleum Company, a corporation of Delaware Application January 10, 1955, Serial No. 480,698

14 Claims. (Cl. Z50-41.9)

This invention relates to mass spectrometers. In one specific aspect it relates to switching means to control the application of accelerating potentials of a first frequency to selected grids of a mass spectrometer.

In recent years mass spectrometers have been developed from highly specialized academic research instruments for measuring the relative abundance of isotopes into analytical tools of extreme sensitivity and accuracy. At the present time, applications are being found for the use of mass spectrometers in process monitoring and control. Mass spectrometery comprises, in general, ionizing ya. sample of material under investigation and separating the resulting ions according to their masses to determine the relative aboundance of ions of selected masses. The material to be analyzed usually is provided as a gas which is bombarded by a stream of electrons to produce the desired ions. Although both positive and negative ions may be formed by such electrical bombardment, most mass spectrometers make use of only the positive ions. These positive ions are accelerated out of the region of the electron beam by means of negative potentials. Such potentials impart equal kinetic energies to ions having like charges such that ions of different masses have different velocities after passing through the electrical eld and, consequently, have different momenta.

The presently known mass spectrometers can be classied in one of two general groups: the momentum selection types and the velocity or energy selection types. The momentum selection instruments sort the ions into beams of different masses by the use of magnetic and/or electrical deccting fields. Ions of selected masses are allowed to impinge upon a collector plate to which is connected an indicating circuit. The velocity or energy selection instruments sort the ions according to the velocities or energies imparted to ions of selected masses by electrical accelerating fields. The present invention is directed primarily toward improving a mass spectrome ter of the latter type. Y

United States Patent 2,535,032 describes a mass spectrometer which is provided with two sets of three equally spaced accelerating grids. Direct potentials are applied to the outer two grids and a radio frequency potential is applied between the center grid and the outer two grids of each set. Ions which enter the space between the first two grids in proper phase receive maximum energy and are accelerated through the field between the first and second grids and the field between the second and third grids. The ions subsequently pass through a fieldfree drift space and enter the second group of three grids. The spacing between the grids, the frequency of the radio frequency voltage, and the magnitudes of the accelerating potentials are such that ions of predetermined mass receive sufficient energy to overcome a potential barrier and impinge upon a collector plate.

The mass spectrometer of the present invention is an improvement over the mass spectrometer disclosed in said Patent 2,535,032. Five sets of accelerating grids are employed to provide four separate driftl spaces. This Fatented Sept. 4, 1956 ICS greatly improves the resolving power of the spectrometer. The radio frequency applied to the accelerating grds'is turned on and olf at a frequency, preferably in the audio range, which modulates the ow of ,ions through the spectrometer tube. An electronic switch actuated by the output 'of a square wave generator is provided for this purpose. The modulated ion current which impinges upon the collector plate is amplified and detected in `a measuring circuit. The stopping potentials and the D. C. step back potentials applied to the grids of the mass spectrometer tube are derived from the output of the radio requency oscillator so that the potentials applied to the grids are maintained constant relative to one another.

Accordingly, it is an object of this invention to provide an improved mass spectrometer which operates upon the principle of energy selection of ions of a predetermined mass.

Another object is to provide a mass spectrometer wherein accelerating potentials of a rst frequency are applied to the ions periodically at a second lower frequency so that the ion beam is modulated at the lower frequency.

A further object is to provide a mass spectrometer of the energy selection type wherein the potentials applied to the several grids of the tube are derived from a common source.

Other objects, advantages, and features of the invention should become apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

Figure l is a .schematic representation of the mass spectrometer of this invention;

Figure 2 is a schematic circuit diagram of the audio oscillator of Figure l;

Figure 3 is a schematic circuit diagram of one embodiment of the radio frequency oscillator and detectors of Figure l;

Figure 4 is a schematic circuit diagram of a second embodiment of the radio frequency oscillator and detectors of Figure l; and' Figure 5 is a schematic circuit diagram of the square wave generator and electronic switch of Figure 1.

Referring now to the drawing in detail and to Figure 1 in particular, there is shown a mass spectrometer tube 10 comprising a gas impermeable envelope, the interior of which is maintained at a reduced pressure by a vacuum pump, not shown, which communicates with the interior of tube 10 through a conduit 11. A sample of gas to be analyzed is directed into tube 1t) through a conduit 12. An electron emitting filament 13 is disposed in one end of tube 10 and an ion collector plate 14 is disposed in the second end of the tube. The end terminals of la- `ment 13 are connected to the respective end terminals of the secondary winding 15 of a transformer 16. The end terminals of the primary winding 17 of transformer 16 are connected to an alternating current source 18.

vThe center tap of the secondary winding 15 of transformer 16 is connected to a negative potential terminal 19.

The gas sample supplied through conduit 12 is directed into an ionization chamber 22 in tube 10 which is defined by a pair of spaced grids 23 and 24 that are maintained at ground potential. Electrons emitted from lament 13 are accelerated into chamber 22 by the potential difference between lament 13 and grids 23 and 24. A grid 26 is positioned between filament 13 and grid 23. Grid 26 is connected to the output of an emission regulator circuit 27, which can be of the type disclosed in the copending application of M. C. Burk, Serial No. 412,790, filed February 26, 1954. The input of emission regulator 27 is connected to the center tap of transformer winding 15. This emission regulator is provided for the purpose of applying a potential to grid 26 of magnitude such as to maintain a constant llow of electrons fr '5 renovar Y into ionization chamber 22 irrespective of Vminor iluctua-V y Y only the gas pressure inthe chamber.

A first collimating electrode 28 is positioned on. the

second'sideofv ionization chamber 22 andi-isA connected to the contactor of,v a potentiometer 20. One end termiV nal offpotentiometer20V is connected tov a negative potential terminal 21', the second end terminal of potentiometer 20 being grounded. A second collimating electrode 128 is spaced :from electrode. 28. Electrode 28? Visconnected to the contactor of a potentiometer 20'.. The` endl-terl minals offpotentiomer 20Y arey connected tofterminal. 21- and ground, respectively. The positive ions producedwin ionization cbamberZZ areaccelerated bythe negative potentials4 applied. to electrodes 28 and 28so as`r to travel through the tube toward l Y. collector plate; 14. n A- iirst Yset ofrthree kequally spacedy grids 35,- 36 and 37 is positioned in tube 10- between.

grid 28: and collector vplate 14;. asecond set of. equally spaced grids 3.8, 39 and 40 isV positioned in spacedV relam tionwith` the first set of. grids; athirdset of equally spaced grids- 42,.43and .44 is positioned in spaced relation with the. second set of grids; a ourth'set. of equally spaced grids 45,46 and 47 ispositionedl in spacedrelationlwith the third set'of grids; and a fth set of` equally spaced grids 49, 50 and 51 is positioned in spaced relation with the fourthv set of grids. Y Grid'35 Vis connectedtor the contactor Vof ay potentiometer 29.` 'One endY terminal. offpotentiomet'e'rr29 isv connected tofa negative 'potential terj minal 30, the second end terminal of potentiometer 29:

beingconnectedto ground. The contactor of potentiometer 29 can he adjusted manually or by a motor 31 to Y apply selected negative accelerating potentials; to grid 35. Grids 36, 39, 43, 46 and 50 are connected to one another and to the output terminal of an electronic. switch 55.

The input terminal ofswitch'55 is connected toone output terminal of a radio frequency oscillator 56. Switch 55 is controlled by the output of a square wave generator 57, whichin turn is energized by the Voutput of an` audio oscillator 58. Switch 55 is thus turned onand oil? at the frequency of osci1lator58v torapply the output of oscillatory 56 to grids36, 39,343, 46 and 50. GridV 35 is connected iive:resistors.64-,f65,.66, 67 and 68 thatare connectedin.

seriesprelation.` V*Gri'ds3'lv andr38-are connected toone another and tothejunction .between resistors. 64- and 65;.v K grids; 40. and 42 are connected toy one another and .to thev junction. between resistors 65 and 66; grids` 44 and 45.-

are connected` to `oneanother. and. toy thevr junctionbetween resistors 66.and 67; andY grids 47 andf 49 areI connected to. one, another and tothe junction between resistors 67 and 68.` .Y

AT plurality of closely spaced. stopping. gridsI 70y is positionedbetween grid. 51 and collector platev 14. Grids. 70

. are connected to one another and to the output` of a stop'- pingde'tector 71, Whichfin turn-isconnectedto an output of. oscillator 56. A plurality Vof suppressor grids 72- isy pos1t1oned between grids 70 andl collectorplateli. Grids l 72v areconnectedv tofa negative potential terminal. 73 to suppress secondary electrons which may result froml ions impinging upon metal parts of the tube. YA. grounded f second inputterminal of amplifier 76 is. connectedto' ground.. The outputY terminals ofamplier 76. are connectedktoV hrst mput terminals of a phase detector 737. The second mputzterminals. of phase detector 7T are con.-

nected'to output terminals of oscillator 58. The output terminals of phase detector 77 are connected to a suitable Y indicating means such'asa recorder 78.

from oscillator 56, the electrical eld between grids 35 and 36 is of such phase that the ionsentering this eld are accelerated. Ions which enter the field during a particular phase of this half'Y cycle' receivev maximumenergy. t During the following halfcycle of the signal fronigoscil-V lator 56, the field between grids 35j `andlis reversed such that; the ions arefurther accelerated. These ions then drift through the held-free space between grids 37 and 38. The masses of theiindividual ionsl determine their Y times of arrival at grid 38.` The ions which arrive at grid 38 at the proper time are again accelerated a vmaximum e 'j Y amountby the. eldappliedlbetweengrids 35i-and` 39, therebyreceiving additionalenergy.y The samev accelerating procedure continues.y asthefions pass through-,the next.` ten grids. lhepositive potential applied tofgrids70- is, v Y adjusted such'that only those ions which receivea. predetermined maximum'energy are able to pas's.-throughf grids', 70 to impinge upon collector .plate 14;L The ionsil.; j impinging upon collector plate 14cause current: to iow Y through the-input-.circuitof amplifier 76' to ground. This current, which is proportional in'magnitude tothe number Y,

Y oi ions impinging upon` the collector'plate, is amplitude` modulatedv at. thesame frequency as audio oscillator 58.

It is this modulated componentrof the current which measured inl accordancewith the. presentinvention` by re cordes 78. The'output voltage from` step back detector j, is. applied`V tofpotential dividing network 63 to decelerate 1 the. ionsso that. the velocity of the selected ions remains substantiallyconstant, whereas the ions'of other masses. are decelerated inpassing through tube 10..

A suitable audio oscillator 58isillustrated schematically-A' in FigureZ; ,This oscillator can beY tuned to provideoutput signals`r of a frequency of approximately 25 cycles per second,V for example. They oscillator comprises. ka. rst triode 80v having.V the anode thereofV connected to aposi-V tive potential terminalSlthrough: a resistor .84.` Y Theconf trol gridof triode- 80 is'. connected to ground through/a i variableresistor. 82 whichisshuntedl by` a capacitor 83. A

Thefcathode of. triode80 is connected to an outputter;

902 is connecter-r to.- groundthrough'a resistor 9.2 which is shunted by a.. capacitor.Y '93., VThe anode of. triode 90' connectedito positive. potential terminal. 8.1through a. resistor 94 andtothe control grid of. athirdtriode 9S through a resistor 96. The control Ygrid of. triodeg9`5 is` connected to ground through a resistor 9.7 andtoy the control. of a fourth. triode 98 through aresistor 9.9..r

Thescathodes of triodes 95'vr and 98 are connected to one another-.and to ground through series: connected. resistorsy 102. and-.88. Thefanodes ofA triodes.95.y and. 98f-aretcon nected to. positive. potentialY terminal1 81; The cathodes f of. triodes.y 95` ande98xarefalso. connectedto output terv j minal throughia capacitor'103. VOutput terminal 85g is'connected, to'fthe. coutrolfgrid. of triode. 80 through aV variable-resistor'105 and afcapacitor 106 whi'chfare con-j nectedin` series relation...V

The, oscillatorof EigureY 2r operates such that;V triodes. Sill-and. 90.. conduct. in unison, as do. triodes: and`f98a- Thesev two pairsV of. triodesiconduct. alternately so that i the. resultingv current. ows. therethrough produceA pulsesfl which initiateroperation oitheo'ther pairrof. triodes. after. j a predetermined time delay. Thelfrequency of.V oscil- ".7 Y latinos. is determined. byfthe. resistance-capacitance network whch provides regenerative coupling, betweenv the:y output oi.. triocles 95. andl 98.- and. the.input. otriode- A80..l

. This regenerative network is formed by resistors 105 and 82 and capacitors 106 and 83. The circuit produces oscillations at a frequency f dened by the following equation:

where Rios, Rs2, Cros and Cas represent values of resistors 105 and S2 and capacitors 106 and 83, respectively. In one embodiment of this invention, resistors 105 and 82 each have total resistances of approximately 716,000 ohms and capacitors 106 and 83 are each 0.01 microfarad. Thus, by varying resistors 105 and 82, which are mechanically coupled to one another, an output frequency of the order to cycles per second can readily be obtained. The amplitude of the voltage appearing between terminal and ground can be adjusted by variable resistor 86.

The particular ion mas detected by the mass spectrometer of this invention is determined in part by the frequency of oscillator 56. In Figure 3 there is shown an oscillator which provides an output signal at a frequency of approximately 3.9 megacycles per second. This frequency is useful in detecting ions having masses in the range of approximately thirteen to one hundred twentyfour. The oscillator of Figure 3 comprises a pentode 110 having the cathode thereof connected to ground through an inductor 111. The control grid of pentode 110 is connected to one terminal of a crystal 112 which oscillates at a frequency of approximately 3.9 megacycles per second. The second terminal of crystal 112 is connected to ground. A resistor 113 is connected in shunt with crystal 112. A pair of capacitors 114 and 115 is connected in series relation between the control gridof pentode 110 and ground. The junction between capacitors 114 and 115 is connected to the cathode of pentode 110. The screen grid of pentode 110 is connected directly to the cathode of a triode 117 which is described in detail hereinafter. 'Ihe screen grid of pentode 110 is also connected to ground through a capacitor 116. The suppressor grid of pentode 110 is connected to ground. The anode of pentode 110 i's connected to one terminal of a tank circuit 118 which comprises a capacitor 119 `and an inductor 120 connected in parallel relation. The second terminal of tank circuit 118 is connected to a positive potential terminal 121 through an inductor 122 and to ground through a capacitor 123. The anode of pentode 110 is connected through a capacitor 125 to the control grid of ia triode 126. The control grid of triode 126 is connected to ground through an inductor 127. The anode of triode 126 is connected to positive potential terminal 121 through a resistor 12S and to ground through a capacitor 129. rllhe cathode of triode 126 is connected through a resistor 130 to one terminal of fa second tuned circuit 131 which comprises a capacitor 132 and an inductor 133 connected in parallel relation. The second terminal of tuned circuit 131 is connected to ground. An output terminal 135 is connected to a point on inductor 133. The oscillations produced by pentode 110 and its associated circuit are ampliied by triode 126 such that the signal appearing between terminal 135 and ground has ya frequency of approximately 3.9 megacycles per second.

The amplitude regulator circuit 59 of Figure 1 is provided to maintain the output voltage from oscillator 56 constant. rihis regulator is illustrated in detail in Figure 3. The junction between resistor 130 :and tank circuit 131 is connected through la capacitor to the cathode of a diode 141 and to the anode of a diode 142. The cathode of diode 142 is connected through a resistor 143 to a terminal 144 which is maintained at a negative potential of constant magnitude by ya suitable voltage regulating circuit, not shown. A capacitor 145 is connected between the cathode of diode 142 and the anode of diode 141. The anode of diode 141 is connected to ground through a capacitor 146. The cathode of diode 142 is also connected through a resistor 147 to the control grid of a pentode 150. The cathode and the suppressor grid of pentode 150 are connected to ground. The screen grid of pentode 150 is connected to positive potential terminal 121 through a resistor 151 and to ground through a resistor 152. The lanode of pentode 150 is connected through a resistor 153 to positive potential terminal 121 and directly to the control grid of triode 117. The anode of triode 117 is connected to positive potential terminal 121.

Diodes 142 |and 141 and capacitors 140 and 145 form a voltage doubling circuit. During the half cycle of the output signal from triode 126 when the cathode thereof is the most positive, capacitor 140 is charged by current ow through diode 142, which current flow also charges capacitor 145. During the following half cycle of the output signal from triode 126, diode 141 conducts current to discharge capacitor 140 and to charge capacitor 145 further to a voltage approximately twice that previously applied across the capacitor. The rectified doubled voltage appearing at the cathode of diode 142 is lapplied to the control grid of pentode 150. If the output voltage appearing at the cathode of triode 126 tends to increase in value, for example, then the rectified Voltage applied to the control grid of pentode 150 is also increased, which increases the current ow through the pentode. The increased current ow results in a greater voltage drop across anode resistor 153, such that the potential applied to the control grid of triode 117 becomes more negative. This negative change in potential decreases the current ow through triode 117 such that the potential applied to the screen grid of pentode 110 becomes more negative. This potential change tends to reduce the output Voltage of the oscillator by an amount suicient to maintain said output voltage constant. If the oscillator output voltage tends to decrease, the potential changes previously mentioned are all reversed such that the screen grid of pentode 110 becomes more positive to increase the output of the oscillator back to the desired value.

Step back detector 61 is illustrated in Figure 3. The junction between resistor 130 and tank circuit 131 is connected through a capacitor to the cathode of a diode 161 and to the anode of a diode 162. rDhe cathode of diode 162 is connected to one end terminal of a potentiometer 163. The contacter of potentiometer 163 is connected to a tirst output terminal 164. The second end terminal of potentiometer 163 is connected through a resistor 166 to a second output terminal 165. A capacitor 167 is connected between the cathode of diode 162 and the anode of diode 161. Diodes 161 and 162 and capacitors 160 and 167 also form a voltage doubling circuit which provides a rectified voltage between terminals 164 and 165.

Stopping detector 71 is illustrated in Figure 3. The junction between resistor 130 'and tank circuit 131 is connected through a capacitor 170 to the cathode of a diode 171 and the anode of a diode 172. The cathode of diode 172 is connected to one end terminal of a potentiometer 173. The contactor of potentiometer 173 is connected to an output terminal 174. The second end terminal of potentiometer 173 is connected through a resistor 176 to ground. A capacitor 177 is connected between the cathode of diode 172 and the `anode of diode 171. Diodes 171 and 172 and capacitors 170 and 177 thus form a third voltage doubling circuit to provide a rectified output voltage between terminal 17 4 and ground.

In Figure 4 there is shown a second embodiment of oscillator 56 which provides an output `signal having a frequency of approximately ten megacycles per second. A frequency in this range is useful in detecting ions having masses up to approximately sixteen. The oscillator cornprises a pentode 180 having the cathode and suppressor grid thereof connected to ground. The control grid of pentode 180 is connected to one terminal of a crystal 181 which vibrates at a frequency of approximately ten megacycles per second. The second terminal of crystal 181 is connected to ground. A resistor 182 is connected in par- `screen grid of pentode 1.8.0. .circuit 19.1 is also connected to'ground through a capaci.-

" of the primary winding 200 of a transformer'201.

Vas a diode.

triode 12.6'V through av capacitorq125f... Triadev 120iV and the. circuit 'as'sociatedtherewith areV similanito the correspondingtriode, andcircuit of Figure 3;1correspon.ding

elements vare designated byy like primed reference nu.-

merals. Detectors 61 .land 71" are also similar tov corresponding'elements in Figure. 3. 'The oscillator` amplitude Aregulator 59. is likewise isimilar to thev corresponding regulator circuitin Figure 3. The anode .of pentode 15.0

iis-.connected to the control grid of atriodel. The

cathode of triode. 190 is connected to one terminal of a.

. tank circuit 191 which comprises 'a capacitor 1.92 connected in parallel with a. variableinductor 193. The s econd terminal of tuned. .circuit llisconnected to.l the Thetirst. terminal of tuned ductor 25.0 tov the movable arm of'switch 230. The junction lbetween c apacitors'f 246 'and 247 is con .tor 195. The. operation of the circuit illustrated. in Figure.

Y 4: is. substantially the ,same as the operationof the Correspondingcircuit in Figure 3. Y f

.The squarewavegenerator 57 and electronic switch 55 are illustrated in Figure 5.V Output terminal 85 of audio oscillator 58 of Figure 2 is -connectedto one end terminal The second end terminal of transformer winding -200 is connected to .ground YOne end terminal of the secondary winding 202 of transformer 201 is connected through a high ohrnic value4 resistor 203 tothe control grid'ofa triode` 204. The second end terminal offtransformer Vwinding 202 and the cathode of triode 204 are connected Vto a. negative potential terminal205. Terminal 205 is connected lthrough -a Ypair of series connected resistors 206 and 207 to a negative potential terminal 208. Terrnif nal 205 is also connected to ground through .a resistor 209. A capacitor 210 is connectedbetween terminal 205 Vandv groundfand a capacitor 211 is connected between ground and the junction between resistors 206 and 207. The circuit comprisingresistors 207 and 206 and capacitors 211 and 210 thusV filters the negative vpotential Yat terminal 208, -which can represent the output of a current rectiiier, not shown. The anode of triode 204 is 'connected'to a positive potential terminal 213 through a high ohrm'cvalue resistor 214. The anode of triode 204 is also connected to the control grid of atriode 215. The 1 control grid of triode 215 is connectedv to ground through aV resistor 216 which 'has a considerably smaller. ohmic value than resistor 2,14. The. cathode of triode. 215 is connected to terminal 205 through a resistor 217. The anode of triode 215 is connected topositive Vpotential terminal 21-3 through series connected resistors 218 and Y21,9. The junction between resistors. 21,8 and 219 is connected. to ground through a capacitor 270 and the anode.

' of triode 215 is connected to ground through a capacitor The. circuit of Figure 5.v thus far described functions to tank circuit: 241 whichcomprises aninductor. 2513.;'Kc-sniffv nectedfin parellelwithj a-capactor 242.. Thesecond terminal Qf tank circuit 241 is vconnectedto the movable arm of switch 235. Terminal 235a lof switch235 is connected to a terminal 230b of switch230. The rstrnentionedterminal of tank .circuit.241 is connected to the cathodes of a double diode 245. The Aanodes of doublek diode 245 are connected to output terminal 135 :ofthe oscillator illustrated iny Figure 3.

135'v lof the oscillator illustrated in Figure 4. The cath-- od'esof-double diode 248 are connected through an in-l nected through a capacitor 251 to an output terminal 252 which is applied to grid's36, V39, 43, 46 `and 50 of mass spectrometertube 10 illustrated in Figurel The electronic switch of Figure 5 functions to apply the output signalsfro'm terminals 135 and: 135' selectively'to `outputterrninal 252fat the same frequency as the signalappliedto the input of thc square wave generator through transformer 201. The'outputsquar'ewave signal from triode215 is applied to the.V anode offdiode 221. tor 220 presents low impedance to thisfaudio frequency signal, 'but presents highirnpedance to. radio frequencyl signals.. The current owV through diode 221. results in a` positivepotential appearing on the cathodethereoi,.which potential is applied through inductor 225, switch 230, and inductor 250 tothe cathodes of diode 24S.. This positive biasing potential prevents current ow through diode 248.

The. square wave outputsignalfromrtriode. 215 is alsorapplied. through switch 235 and inductor'243 to the'cathode 252. The cathodes of diode245 are connected tog'round through tank circuit 2.411, switch'235 landra capacitor 234.

' However, circuit241 is tuned to the same frequency as.

generate substantially square waves of the same frequency as the sinusoidal output of oscillator 58. The high ohmic value resistors 203 and 214 function to limit the current iiow through triodes 204v and 2 15 such that the potential appearing at the cathode of triode 215 varies in substantially a square wave form.

. The cathode of triode k215 isY connected through an inductor 220 to the anode of a triod'e 22,1 which functionsas a diodefbecausc the control grid thereof is connected tothe anode. Triode 221 will be'referred to hereinafter The cathode of triode 215 is also connected to ground through a capacitor 234. The cathode of diode 221 is connected to ground through av resistor 223 which is shunted by a capacitorv224. The cathode'of diode. 221

which is'cormected to terminals 230a and 23517 of respective switches 230 and 235. The anode. of diode 221is connected through a capacitor 2,40 to one'tcritnirlalof.` a

`is also connected through an inductor 225.v to a lead-226 v the signal appliedto. terminal frorn'the oscillator of Figure .3 and thus presents high impedance to thissignal. The radio frequency current ow through diode 245 is applied' to` terminal v252 through capacitors 246 and1251;V

Y When it is desiredto apply Vthe signal at terminal 135"A fromthe'oscillator of Figure'4 to output terminal 252,

switches 230 andV 235 are moved to their right-hand posi vents currentl flow` throughdiode 245. The square wavesignal from triode 2515 is applied through switch 230`and inductor 250m the cathodes of diode 248. This squarewave signal permits current'fiow through diode 248rdur ing alternate half cycles. .The radio frequencypcurrent ow through diode 248 is thus applied to ouput terminal 252 through .capacitors '247 and 251. Inductor 250 has stray capacitance in parallel therewith which forms a secondtank circuit; Y

output terminals 164 and-164' of Figures 3 and 4 `aire connected to the contacts 260tz`and 260b, respectively, of a switch 260. .Switch arm 260 is connected to'a'n'out-- put terminal 264. Terminals 174 and'174 Vof Figures 3 and 4 are connected to the contacts 26111 and 261b, respectively, of switch, 261.' Switch armv 261 is connected to. an output terrnhxalZ'l. The movable arms of switches 260,1 261, 230 and 235 are mechanically coupled toone f another as.` illustrated.. Terminal 264-is connected to gridi 51 of rnass. spectrometer tube 10;terminals 165 and 165' i i The iirst terminal of" Y tunedcircuit 241 is also connected through 'a pair of series connected capacitors 246'an`d 247 to5t-hecathod`es 6i Y second double diode. 248. The anodes. of double ,diode 248 Vare connected to one another and'tooutput .terminal Induc- Q v of ions of different mass ranges by moving the arms of these switches.

The over-all operation of the mass spectrometer of this invention should now become apparent. With reference to Figure 1, electrons emitted from heated lament 13 are accelerated into ionization chamber 22. The electron ow into this ionization chamber is maintained constant by emission regulator 27, which is desirable but not essential to operation of the mass spectrometer of this invention. The positive ions formed in chamber 22 are accelerated toward collector plate 14 by the negative potential applied` to grid 28. Ions of a particular mass receive maximum energy as they pass through the radio frequency fields applied to the next fteen grids in the tube. The ions which acquire suicient energy to overcome the positive potential barrier applied to grids 70 impinge upon collector plate 14 and are detected by the circuit connected thereto. The amplifier 7 6, phase detector 77 and recorder 78 preferably are of the type described in the copending application of M. C. Burk, Serial No. 431,805, filed May 24, 1954, although other circuits to measure the component of the output signal of the same frequency as oscillator 58 can be employed.

In the operation of the mass spectrometer tube of this invention it is important that a fixed relationship be maintained between the radio frequency accelerating voltages and the D. C. step back potentials. It has been found that the use of a square wave signal to actuate electronic switch 55 is advantageous. If a sinusoidal signal were employed for this modulation, then the radio frequency signal envelope Would vary in a sinusoidal fashion. Under such circumstances the radio frequency potential at which the tube is designed to operate would be realized for only a very short interval at the peak of the envelope. The square wave signal, however, permits the radio frequency envelope to remain at the proper level for substantially one-half of the period. This permits an output signal of greater magnitude from the collector plate than could be obtained with the use of a sine wave.

In the mass spectrometer tube of Figure 1, the spacings s between grids 35 and 36, 36 and 37, 38 and 39, 39 and 40, 42 and 43, 43 and 44, 45 and 46, 46 and 47, 49 and 50, and 50 and 51 are maintained equal. The spacings r between the centers of grids 37 and 38, 40 and 42, 44 and 45, and 47 and 49 can be represented by the expression:

where n is an integral number, all dimensions being in inches. In one embodiment of this invention, s was 0.118 inch and the four ns, proceeding from lament 13 to collector plate 14, were live, nine, four and seven, respective ly. The values of n can be varied, they may be equal, and more or fewer drift spaces can be provided, if desired.

The circuit components of oscillator 58 were as follows: resistor 84, 470,000 ohms; resistor 99, 470 ohms; resistor 94, 180,000 ohms; resistor 92, 220,000 ohms; resistor 96, 560,000 ohms; resistor 97, 820,000 ohms; resistor 102, 10,000 ohms; resistor 88, 56 ohms, resistor S6, 3,250 ohms; capacitor 91, 0.02 microarad; capacitor 93, 8 microfarads; capacitor 103, 25 microfarads; triodes 80 and 90, each one-half of a tube type 12AX7; and triodes 95 and 98, each one-half of a tube type 12AU7. The voltage supplied to terminal S1 was obtained from a terminal at approximately 450 volts through lter resistors of 10,000 ohms. The values of resistors 82 and 105 and capacitors 83 and 106 are described heretofore.

In Figure 3, the circuit components were as follows: resistor 113, 47,000 ohms; resistor 153, 470,000 ohms; resistor 151, 68,000 ohms; resistor 152, 10,000 ohms; resistor 128, 5,600 ohms; resistor 130, 330 ohms; resistors 143, 147 and 176, 1,000,000 ohms each; resistor 166, 175,000 ohms; potentiometer 163, 50,000 ohms; potentiometer 173, 300,000 ohms; crystal 112 vibrates at 3.9 megacycles per second; capacitors 116, 123, 145, 167 and 177, 0.0033 microarad each; capacitor 114, 10

micro-microfarads; capacitor 115, 220 micro-microfarads; capacitor 119, 68 micro-microfarads, capacitor 125, 100 micro-microfarads; capacitors 129 and 136, 0.01 micro'- farad each; capacitor 132, 300 micro-microfarads; capacitors 140, 160 and 170, 100 micro-microfaradseach; capacitor 148, 0.1 microfarad; pentodes and 150, tube type 6AU6 each, triodes 117 and 126, each one-half of a tube type 12AU7; diodes 161, 162, 141, 142, 171 and 172, each one-half of a tube type 6AL5; inductors 111, 122 and 127, 2.5 millihenries each; inductor 120, 45 turns of No. 28 wire on an )CR-50 form;'and inductor 133, 30 turns of No. 24 wire on an XR-SO form, tap 135 being seven turns from the bottom. Terminal 121 was maintained at 300 volts, and terminal 144 was maintained at -90 volts.

In Figure 4, the circuit components were as follows: resistor 102, 100,000 ohms; resistor 128', 5,600 ohms, resistor 130', 330 ohms; resistor 153', 470,000 ohms; resistor 151', 68,000 ohms; resistor 152', 10,000 ohms; resistors 147 143' and 176', 1,000,000 Ohms each; resistor 166', 175,000 ohms; potentiometer 163', 50,000 ohms; potentiometer 173', 300,000 ohms: capacitors 192 and 134, 47 micro-microfarads each; capacitors 195 and 196, 0.005 microfarad each; capacitor 125', 100 micromicrofarads; capacitors 129', 146', 148' and 168, 0.01 microfarad each; capacitors 160', 170', 145', 167 and 177', 100 micro-microfarads each; crystal 181 vibrates at ten megacycles per second; pentodes 180 and 150' and diodes 141', 142', 161', 162', 171' and 172', same as their counterparts in Figure 3; triodes 190 and 126', each tube type 6C4; inductor 127', 2.5 millihenries; and inductors 193, and 133', each 12 turns of No. 18 wire on an X11-50 form tap 135' being three turns from the bottom of inductor 1.33'. Terminal 121' Wasrnaintained at 300 volts and terminal 144' was maintained at -85 volts.

`In Figure 5, the circuit components were as follows: resistors 203 and 214, 1,000,000 ohms each; resistors 218 and 219, 8,200 ohms each; resistor 216, 68,000 ohms; resistor 217, 15,000 ohms; resistor 223, 10,000 ohms, resistors 206 and 207, 15,000 ohms each; resistor 209, 20,000 ohms; capacitors 210 and 270, 20 microfarads each; capacitor 211, 40 microfarads; capacitors 246, 247, 234 and 251, 0.01 microfarad each; capacitor 240, 0.001 microfarad, capacitor 224, 50 microfarads; capacitor 242, 82 micro-microfarads; inductor 220, 2.5 millihenries; inductor 250, four turns of No. 18 Wire on an XR-50 form; inductor 243, thirty turns of No. 24 wire on an XR-SO form; inductor 225 was a choke coil formed of No. 34 Wire on a one-half inch form; triodes 215 and 221, each one-half of a tube type 12AU7; triode 204, tube type 6AB4; and diodes 245 and 248, each tube type 6AL5. Terminal 213 was maintained at 300 volts, and and terminal 20S was maintained at -600 volts.

In Figure 1, the circuit components were as follows:`

resistors 64, 65, 66, 67 and 68, 120,000 ohms each, and potentiometer 29, 300,000 ohms. The potentials applied to terminals 73 and 30 were approximately `600 volts, and the potential applied to terminal 19 was approximately 60 volts. Adjustment of potentiometer 29 enables ions of selected masses to impinge upon collector plate 14.

From the foregoing description it should be apparent that there is provided in accordance with this invention an improved mass spectrometer. The instrument is particularly useful for process analysis and control because of its small size. The spectrometer does not require a magnetic deecting field and is therefore less bulky and less expensive to operate than conventional mass spectrometers. While the invention has been described in conjunction with present preferred embodiments, it should be evident that the invention is not limited thereto.

What is claimed is:

1. A mass spectrometer comprising a gas impermeable envelope enclosing a source of ions, a collector plate spaced from said source of ions, a plurality of groups of grids .spaced in a line between said source of ions and said plate, each of said groups comprisingv three grids Y in spaced relation with one another, the spacings between ladjacentrgrids being equal, the spacin'gs between adjacent groupsof said grids being substantially inches, where n is an integral number and s is the spacing Ybetween adjacent grids in each group, all of said dimem 'sions being in inches,l and a second grid positioned between said collecton plate and said groups of grids; means 2 applying Ysteady potentials to the twoend grids in each` of said groups of grids; means applying a poten- 'tial to said. second grid of polarity opposite the polarity of the ions being detected; a source of potential .which luctuates in. magnitude at a -rst frequency; switch- "`ing. means connecting `said source of potential to the Y center grid in each of said groups of grids; means. to Vactuate .said switching means at a secondV Vfrequency potentiometer to the cathode of said second diode, means.,

connecting the second 'endV terminal of said potentiometer to .the anode of said rst vdiode,.and means connecting the contactor of Vs aidpotentiorneter tosaid secondV grid.-

v..7.'I'l1e combination-in accordance with claim 1 where-v in said means Vapplying steady potentials'rto the two endA grids .in each-,of said groups'of grids comprisesfcurrent whereby said source of potential is applied to the center.

grid in each of said groups at said second frequency;

adapted. to pass vcurrent from said source of potential when energized by a voltage Vof predetermined magnitude, means producing a uctuating voltage of substantially sinusoidal wave form at said second frequency, a

of said square wave generator to said switch.

3. The combination in accordance with claim 2 where-v in said switch comprises a diode, means connecting` the anode of said diode to said source of potential, means Yconnectingthe cathode of said diode to the center grids of each ofsaid groups of gridsgsaid diode normally being Vnon-conductive, and means applying the output of said square wave generator to thecathode of said diode, the magnitudejof the output signal of said square wave beingr of such magnitude to render said diode conductive during alternate half cycles.

4. The combination in accordance withclaimVV 3 furtherA Vcomprising a s econd source ofl potential which uctuates in magnitude at a third frequency, a second diode, means connecting the anode of said second diode to said. second source of. potential, means connecting the cathode of said.. second diode to the center grids of. each of said groupsV of grids,"a sourcev of bias potential, and switching means to apply said. bias. potential selectively to the-cathodesof said diodes and to apply the output.y of said: square wave generatorjselectively to the cathodes of opposite'onesof v said diodes.

5. Thecombination in accordance with claim-14 where-4 in saidrneans applyinga potential to saidY second grid y of polarity opposite the polarityV of the lions being detected Vcomprises current rectifyingvmeans, and meansg connecting. said source of potential to the input of said current rectifying means.. i

6. The combination in accordance. with claim 5 wherein said current rectifying meanscomprisesral rst diode, a Second diode, a rst capacitor having onerterminalv rectifying means,y means connecting said source of ypoten tial to the input of said current rectifying means, a voltage'dividing network, and means applying the output of said current rectifying means across said Voltage dividing network, said steady potentials being obtained. fromjsej lected points on said voltage dividingrnetwork'.Y

Y Y l8. The combination inraccordance with claim 7 wherein said current rectifying means comprises a. firstdiode, a second diode, a yfirst capacitor having one terminal thereof connected tothe cathode of said tirst diode andY to the anode of said -second diode, the' second-terminal of said rst capacitor being' connected to said source of potential, a second'capacitor havingone ternziinaltlrl'ere-v of connected to the anodel of said first diode 'and the second terminal thereof Yconnected `to -the cathode of said second diode, the irst terminall of said*v second capacitor also being connected `to a point of reference potential, a potentiometer, means connectingV one endV Y terminal of said potentiometer to the cathode of said second diode, means connecting the second end terminal of said potentiometer to the anode of?l said rst diode,

means connecting the contactor of said potentiometer to one end terminal of said voltage dividing network, and means connecting said point of reference potential to the second end terminal of` said voltage dividing network.

9. The combina-tion in accordance with claim l wherein said means applyingV a potential to said second grid of polarity opposite. thepolarity ofthe ions being de-v tected comprises first current rectifying means, and vmeans connecting said source of potential to the inputV of vsaid rst V*current rectifying means, and wherein said means applying steady potentials to, the two end grids lin eachof said groups of grids comprises second currentrectifying means, means connecting said source yof potentiall to the input of said Ysecond current rectifying means, a yoltf, age dividing network, `and means applying the output ofA chamber, means applying a potential ditference between said lament and said chamber to'direct electrons lfrom said lamen-t into said chamber, a collector plate. spaced' from said chamber, a irst grid positioned between-said chamber and saidA plate, means applying a rstpotential to said first grid to directionsv from saidchambertoward said plate, a plurality of groups of second grids spaced in a line between said tirst grid and said plate, each ofV said groups comprising three grids in spaced rela tion with. one another, lthe spacings between adjacent grids being equal, the spacings 'between 'adjacent` groups thereof connected to the cathode of said first dioderand to the anode of said second diode, the seconduterminalV of said first capacitor being connected toV said source of potential, a second capacitor having oneterminal there.- of connected to the anode of saidriirst diode and thesec ond terminal thereof connected ot the-cathodeof said second diode, the rst terminal of said secondrcapacitor also being connected to a point of referencev potential, a Ipotentiometer, means. connecting. oneend terminal' oisaidl of said grids being substantiallyl Y Y n i r' l zshmz'l) inches, where n is. an Yintegral number and .sV is the spacing between adjacent' grids in each group, nall of said` dimensions being ininches, a Vthird lgrid positioned between said. groups of second grids andv said plate; a

voltage dividing network,..the two end grids in each of said groups of second. grids` being connected'fto selected points on said network; meansiapplying. a second potential across said voltage dividing network; means applyingl a third potential to said third grid of polarity opposite'the potential of rst frequency; switching means connecting said source of alternating potential to the center grid in each of said groups of second grids; means to actuate said switching means at a second frequency whereby said source of potential is applied to the center grid in each of said groups at said second frequency; and means connected to said collector plate to measure ions impinging thereon at said second frequency.

ll. The combination in accordance with claim l0 further comprising iirst and second current rectifying means, and means connecting said source of alternating potential to the inputs of said current rectifying means, the output of said rst current rectifying means supplying said second potential, and the output of said second current rectifying means supplying said third potential.

12. The combination in accordance with claim 1l wherein each of said current rectifying means comprises a rst diode, a second diode, a rst capacitor having one terminal thereof connected to the cathode of said first diode and to the anode of said second diode, the second terminal of said rst capacitor being connected to said source of alternating potential, a second capacitor having one terminal thereof connected to the anode of said iirst diode and the second terminal thereof connected to the cathode of said second diode, the iirst terminal of said second capacitor also being connected to a point of reference potential, a potentiometer, means connecting one end terminal of said potentiometer to the cathode of said second diode, and means connecting the second 14 end terminal of said potentiometer to the anode of said first diode.

13, The combination in accordance with claim 10 further comprising a second source of alternating potential of a third frequency, second switching means to apply said second source of alternating potential to the center grid in each of said groups of grids selectively in place of said rst-mentioned source of alternating potential, and

means to vary the magnitude of said first potential applied to said first grid.

14. A mass spectrometer comprising a gas impermeable envelope enclosing a source of ions, a collector plate spaced from said source of ions,a plurality of first grids spaced in a line between said source of ions and said plate, a second grid positioned between said collector plate and said plurality of first grids, means applying a -potential to said second grid of polarity opposite the polarity of the ions being detected, a source of potential which fluctuates in magnitude at a rst frequency, switching means to connect said source of potential between selected pairs of said first grids, means to actuate said switching means at a second frequency so that said source of potential is applied between said selected pairs of iirst grids at said second frequency, and means connected to said collector plate to measure ions impinging thereon at said second frequency.

No references cited. 

1. A MASS SPECTROMETER COMPRISING A GAS INPERMEABLE ENVELOPE ENCLOSING A SOURCE OF IONS, A COLLECTOR PLATE SPACED FROM SAID SOURCE OF IONS, A PLURALTIY OF GROUPS OF GRIDS SPACED IN A LINE BETWEEN SAID SOURCE OF IONS AND SAID PLATE, EACH OF SAID GROUPS COMPRISING THREE GRIDS IN SPACED RELATION WITH ONE ANOTHER, THE SPACINGS BETWEEN ADJACENT GIRDS BEING EQUAL, THE SPACINGS BETWEEN ADJACENT GROUPS OF SAID GIRDS BEING SUBSTANTIALLY 